1. Field of the Invention
The present invention relates to a support structure for supporting gears of a gear train for power transmission.
2. Description of the Related Art
In drive mechanisms for moving an object to be driven by translating rotary motion of a rotary output shaft of a drive source such as a motor into axial (linear) motion, two types of drive mechanisms are known in the art: a drive mechanism which directly connects a lead screw (leadscrew) to the rotary output shaft, and a drive mechanism which transfers rotational torque from the rotary output shaft to another lead screw via a gear train. The former drive mechanism tends to be long in the axial direction of the rotary output shaft and thus unsuitable for an apparatus which needs to be slimmed down in the axial direction. In the latter type of drive mechanism that uses a gear train, the drive mechanism can be slimmed down in the axial direction because a drive source such as a motor and the lead screw can be arranged side by side. However, in the case of using a gear train, heed has to be given not only to the support stability of the driving gear on the rotary output shaft side and the support stability of the driven gear on the lead screw side but also the support stability of the intermediate gear positioned between the driving gear and the driven gear. For instance, in an AF (autofocus) mechanism for an imaging device such as a digital camera, gears of the gear train are susceptible to rattling and produce noise if the support for the intermediate gear has excessive play because the motor is driven intermittently at high speed. An example of this type of support structure for supporting the intermediate gear of a gear train is disclosed in Unexamined Japanese Patent Publication 2007-114530 (hereinafter referred to as Patent Document 1).
In Patent Document 1, a spring washer (disk spring) is inserted between an end of a spur gear which includes an intermediate gear of a gear train and a support member which supports the rotational shaft of this spur gear to stabilize the spur gear by the biasing force of the spring washer. In general, a variation in resiliency (variation of load) of this type of biasing member per unit of displacement magnitude becomes greater as the size (distance from the point of support to the force-applied point) of the biasing member becomes smaller, and therefore, it is desirable that the size of the biasing member be as large as possible in order to obtain a stable biasing force in the case where dimensional tolerances of individual parts are taken into account. Various outer shapes of the spring washer disclosed in Patent Document 1 such as a circular shape, an oval shape and a rectangular shape have been proposed. If the spring washer with this kind of shape is increased in size, the distances between the gear shafts of the intermediate gear and the associated gears in front and in the rear of the intermediate gear need to be increased to prevent these gears from interfering with the spring washer, which becomes a cause of increasing the size of the gear mechanism.